Federal and state air quality standards on emissions have made it necessary to seek methods of cleaner burning for carbonaceous fuels. Both SO.sub.2 and NO.sub.x are legislatively regulated. The differences in the behaviour of these pollutants and the differences in the best available methods for independently eliminating these gases make an existing single-stage combustion process for their co-removal highly unlikely or at least inefficient.
Fluidized bed combustion has developed recently for burning sulfur-bearing, carbonaceous fuels. This is primarily due to the ability to contact the sulfur dioxide reaction product with a sulfur sorbent for a period of time sufficient to capture a high percentage of the sulfur dioxide. Work on nitrogen oxide removal has trailed the work on sulfur oxide capture because of less stringent legislation to this point.